1. Field of the Invention
This invention relates to a head rest apparatus, and more particularly to a head rest apparatus including a driving mechanism for moving a head rest main body by a nut and a screw.
2. Description of the Related Art
A head rest apparatus is disclosed in Japanese Utility Model Laid Open Publication No. 2(1990)-61233 published without examination on May 8, 1990.
This head rest apparatus includes a stay means fixed on a seat back, a head rest main body movably supported on the stay means in the vertical direction, a bracket disposed in the head rest main body and fixed to the stay means, a screw fixed in the head rest main body, a nut rotatably supported on the bracket and engaged with the screw, and a motor disposed in the head rest main body and connected to the nut for moving a head rest. The head rest main body is moved relative to the seat back in response to the movement of the screw by the nut which is in turn rotated by the motor. Therefore, a position of the head rest main body is adjusted relative to the seat back.
However, in the conventional structure as mentioned above, if a force is applied excessively to an engaged portion between the nut and the screw, the engaged portion will be damaged, thereby disturbing further smooth relative movement between the nut and the screw, which leads to a malfunction of the height adjustment of the head rest apparatus.